In gas turbines engines, a number of discs including radially extending blades which are inserted to the discs are provided to form a rotor. There are sets of discs for compressor blades and sets of discs for turbine blades. The respective sets of discs are retained by a turbine nut and a compressor nut respectively applied to one or two tension studs, the nuts and the studs are used to apply a preload to tension the arrangement to ensure that all rotating parts are secure during operation of the turbine.
In current turbines, the rotor may be held together by a pair of tension studs. In the following one possible way how to assemble a compressor and a turbine is explained in a simplified manner. A first threaded end of the first stud may engage into a threaded bore in a shaft element of the rotor. A compressor disc then may be pushed axially into position and locked to the shaft element. Further compressor discs may additionally be pushed into position. Finally a threaded compressor nut may be engaged to a second threaded end of the first stud and tightened such that all compressor discs are secured to each other and the shaft element. For the turbine discs, a first threaded end of the second stud may engage in a threaded bore of the other end of the shaft element. Then turbine discs may be pushed axially into position from the opposite side and a threaded turbine nut may be applied to a second threaded end of the second stud and tightened such that all turbine discs may be locked to the shaft element.
A prior art gas turbine arrangement is known from UK patent application GB 2452932 A and is also shown in prior art FIG. 1 which is a longitudinal section along an axis of a bladed rotor of a gas turbine, the axis being an axis of rotation. It comprises—left to right looking at the figures—, an axially extending compressor stud, a compressor nut, an inlet shaft, a set of compressor discs, an intermediate shaft, a turbine stud, a set of turbine discs and a turbine nut. In FIG. 1 shows different stages of assembly of the gas turbine arrangement. Please note that the order of assembly may be different to the sequence as explained in the following.
In FIG. 1A, a threaded compressor stud is rotated into threaded engagement into a threaded bore in an intermediate shaft and compressor discs are slid over the compressor stud from left to right during assembly. An inlet shaft is then mounted onto the compressor stud and a compressor pre-load nut threaded onto the compressor stud end. A hydraulic tool is applied to stretch the stud and the compressor nut is tightened to engage the inlet shaft before the tool is removed. This retains the pre-load applied to the compressor stud. The stretch required may be affected by relative thermal and mechanical expansion and contraction at different operating conditions of the stud and the clamped components.
FIG. 1B shows a turbine stud threaded into another axial end of the intermediate shaft. Then—not yet shown in FIG. 1B—the next stage is to assemble the turbine discs onto the turbine stud from right to left with a turbine nut being threaded onto the other end of the turbine stud, as shown in FIG. 1C. The hydraulic tool is applied to stretch the turbine stud and the nut is tightened to retain the pre-load when the tool is removed.
It will be appreciated that this is a complicated arrangement which requires careful machining and assembly for adequate operation and a long service life. The material of the stud, the dimensions of the stud, the amount of stretch of the stud, etc. has to be considered to ensure sufficient rotating load at all operating conditions of the gas turbine engine. In particular, the threaded connections and the studs may experience stress. It has to be noted that with “load” a clamping force is meant applied by the stud to the discs.
From patent GB 898153 a shaft is known that consists of two pieces that are assembled together via a thread. Both pieces have different diameters. From WO 2004/076821 A1 an air thrust bearing is disclosed in a gas turbine engine having a single piece shaft comprising of two sections having a slightly different diameter.
In EP 0742634 A2 a compound shaft is disclosed. A first stiff shaft is connected to a second stiff shaft via a flexible disk shaft.
According to US patent U.S. Pat. No. 3,612,628 a bolt comprising of two sections can be inserted into a hollow single piece rotating shaft of a bearing.
It is a goal of the invention to reduce stress and fatigue of the stud and the threads.